1. Field of the Invention
The present invention relates to a process for fabricating a membrane structure and more particularly to a process for fabricating a silicon carbide membrane for an X-ray mask having improved radiation hardness.
2. Description of the Prior Art
U.S. Pat. No. 4,543,266, issued Sep. 24, 1985 to Matsuo et al entitled METHOD OF FABRICATING A MEMBRANE STRUCTURE, discloses a process wherein a thin film which becomes a membrane is formed over one major surface of a substrate by a plasma deposition process utilizing microwave electron cyclotron resonance. The substrate is then removed, other than a portion of the substrate which remains as a frame, so as to form a membrane structure. A dense and high quality membrane is formed at a low temperature and the internal stress of the membrane is controlled by varying the conditions under which the plasma deposition process is carried out and by heat treating the thin film after its formation. The heat treatment is related only to stress control on the films. There is no teaching of improvement of radiation hardness and the thermal treatment described is not associated with minimizing radiation damage.
European Patent EP 372645A, issued Jun. 13, 1990 entitled X-RAY MASK MEMBRANE OF SILICON-CARBIDE, discloses a structure wherein, at least one of the two main surfaces of a single crystalline Si wafer, a SiC layer is formed. This is made into a mask-supporting membrane by removal of Si-material over the entire surface except the edge-regions. The feature is that before or after this selective etch, an anneal takes place at a temperature in the range of 200-1350 degrees C. for 2-10 hours in an oxidizing ambient. The preferred conditions are four hours at 1100.degree. C. This anneal may also be given to selected areas of the SiC film using laser-beam heating in an oxidizing ambient. The SiC film may be formed by CVD on the wafer substrate. The process can produce layers with a lower stress, a higher transparency to visible light used for alignment and a lower surface roughness.
The annealing is used exclusively for stress control and the application of an antireflective oxide coating. The heat treatment is done under oxidizing atmospheres to apply the oxide coating. There is no mention of radiation damage avoidance.